VERITAS Volume Manager 4.1 Administrator's Guide

Performance Monitoring and Tuning

Tuning VxVM

Chapter 12 409

The default for this tunable is 50 ticks.

Increasing this value results in slower recovery operations and

consequently lower system impact while recoveries are being performed.

vol_fmr_logsz

The maximum size in kilobytes of the bitmap that Non-Persistent

FastResync uses to track changed blocks in a volume. The number of

blocks in a volume that are mapped to each bit in the bitmap depends on

the size of the volume, and this value changes if the size of the volume is

changed. For example, if the volume size is 1 gigabyte and the system

block size is 1024 bytes, a vol_fmr_logsz value of 4 yields a map

contains 32,768 bits, each bit representing one region of 32 blocks.

The larger is the bitmap size, the fewer the number of blocks that are

mapped to each bit. This can reduce the amount of reading and writing

required on resynchronization, at the expense of requiring more

non-pagable kernel memory for the bitmap. Additionally, on clustered

systems, a larger bitmap size increases the latency in I/O performance,

and it also increases the load on the private network between the cluster

members. This is because every other member of the cluster must be

informed each time a bit in the map is marked.

Since the region size must be the same on all nodes in a cluster for a

shared volume, the value of the vol_fmr_logsz tunable on the master

node overrides the tunable values on the slave nodes, if these values are

different. Because the value of a shared volume can change, the value of

vol_fmr_logsz is retained for the life of the volume or until FastResync

is turned on for the volume.

In conﬁgurations which have thousands of mirrors with attached

snapshot plexes, the total memory overhead can represent a signiﬁcantly

higher overhead in memory consumption than is usual for VxVM.

The default value of this tunable is 4 KB. The maximum and minimum

permitted values are 1 and 8 KB.

NOTE The value of this tunable does not have any effect on Persistent

FastResync.